I know I'm being stupid, but it seems a bit disloyal of Terry to drop Rinso, when he was the one who got him started in the first place. Perhaps just one more book, to make sure he is OK. Rincewind that is, not Terry.

Unlikely, sadly.Rinso appeared in UA with a role far too small to be of importance and yet too big for a mere cameo. And what he had there were a few lines rather leeching on his reputation (granted, there was one that ranks as my absolute favourite lines in Discworld history...well...the condensed version at least, but still) than actually Rincewind being Rincewind.

There might, in addition, be other reasons for him not being featured centrally again as well, but those would only be speculation.

I think all the wizards now bore Terry to tears as they're not as interesting as the Witches because they don't interact with people enough (or properly). Their future has really been more in the Science of the Discworld series which allows their more analytical and academic posturings to come out more effectively and humourously to off-set the actual workaday populist science that's really being explores

Rincewind's confirmed role as the Professor of Cruel and Unusual Geography and his familiarity with surviving in arcane and hostile environments, including potentially L-Space, make him the paramount focus for that line of Disc development

"Some men see things as they are and ask why. Others dream things that never were and ask why not.” George Bernard Shaw

I think the witches bore him too, seeing he gave Granny a perfect little kitty.And the wizards have been reduced to grumpy old grampas with no idea of reality and the life outside the university walls. Even Ponder who before had more than just an idea of how real life worked now....I stop here...

It's quite possible that Pterry has become bored with the entire DW series and wants to shake himself free of it. After 39 books the man is entitled to move on.

His editorial calendar shows him focusing on other, non-DW novels (I don't count the 'spin-off' stuff as real DW output) for at least this year, and, as others have said, his last couple of DW books (Snuff and ISWM in particular) do seem a bit valedictory in nature.

Nation is very good. Even better subsequent reads. I like Rincewind. Have enjoyed all the novels he's been in. I don't get this early, classic, post Night Watch era thing. Night Watch, The Fifth Elephant & Going Postal etc, are exceptional, but I enjoyed earlier books like Witches Abroad, Lords & Ladies & Reaper Man just as much.